Post gets deleted if it contains “Oecd x”

We are facing a mysterious bug.

Writing a post on Oecd, as soon as we would hit save firefox, would go to the post page: /wp-admin/post.php
But giving a file not found page.
By cutting the post in various chunk we discovered that wordpress does not admit any post that include the string:
“Oecd x” where x is any letter, and there is ONLY one space between the d and the x. Eventually we manage to post by using “Oecd<space><space>x”.

What’s the root of the problem? Seems really weird.

Many thanks,
Pietro